Optical fiber networks are the backbone for today's most important communication technologies. Optical signals generated by, e.g., infra-red lasers carry all manner of data across these networks by high speed modulation of the intensity of their light, decoded subsequently into the now familiar 1s and 0s of digital information. The last twenty years resulted in billions of dollars of investment in the placement of such fiber networks, often in locations not easily accessible like ocean floors and underneath roadways. Most of this fiber is single mode, i.e., allowing the passage of light only within a narrow spectral window, so that earlier attention to increasing the capacity of these networks focused on ever faster modulation and detection of the laser signals. The transmission bandwidth of single mode optical fiber is wide, but not infinitely so, and thus suggests another approach to increasing its capacity: Propagation of several signals simultaneously, each carried by the modulation of light at a discrete wavelength. This approach, called wavelength division multiplexing (WDM), is also possible albeit technically more difficult to achieve. The key requirement here is that the propagating light be confined to well defined and stable wavelengths with a small spectral dispersion (ideally <0.2 nm) with the constraint that all of the conjoining signals still reside within the spectral bandwidth of the fiber, is achievable by exotic methods. Practical implementation of WDM awaits technologies that provide compact, manufacturable arrays of lasers, separated spectrally by a deterministic frequency (typically 200–400 GHz depending on the fiber), at relatively low cost.
The requisite wavelength definition and stability can result from single frequency lasers using distributed feedback resonators (DFR). The latter, when temperature-stabilized, provide the resistance to wavelength drift and spectral purity necessary for practical implementation and are well known in research and already widely used in this field. Achieving this wavelength accuracy in arrays of DFRs, however, requires an additional step in their manufacture. The needed periodic gratings must have phase-shifts in the period of the grating, centered along the length of the laser cavity, that lock and stabilize the propagating field, with each grating in the array precisely altered in pitch with respect to its neighbor. This alteration in pitch results in a deterministically different wavelength for each laser within the array (true for either multi quantum well type or bulk active mode lasers although the former may offer advantages of manufacture). Assuming adequate control is achieved over the epitaxial film thickness and its composition, accurate and precise definition of the grating pitch is the limiting requirement in the successful manufacture of arrays of DFR lasers. It has to be noted that the requirements in the size of the features are generally below those accessible using conventional optical lithography.
One manufacturing approach for the placement of these gratings on laser mesas uses near-field holography with masters formed by e-beam lithography. The latter provides the necessary resolution and accuracy in pitch and placement of the wavelength phase shifter, while the former has the convenience of parallel formation of features while maintaining high resolution. Here, a grating array in silica results from e-beam lithography to provide a master with typical feature size of about 100 nm at periods of 200 nm, necessary for implementation of DFR lasers at wavelengths of 1.5 μm. An image of the master grating is printed by its exposure, using UV light, of photoresist in contact with the master. Contact transfer of the pattern by near field holography is necessary because it allows the very high resolution required in the formed structures in a manner not practically accessible by projection lithography. This approach, however, suffers from several disadvantages. First, the masters are very expensive to produce, costing $15,000 (about 10 times the cost of conventional masks) and have limited lifetimes (about 50 uses) due to damage and obfuscation of their features under the stresses of mechanical contact with the substrate of the putative array. Second, changing the pitch of the gratings requires the formation of a new master so that compensating absolute changes in the design of the laser mesa requires wholly new grating masters even for slight changes in the targeted spectral range. Third, pattern transfer printing must occur everywhere at once on a wafer. The stresses associated with printing large areas by brittle contact between the silica grating and resist can further erode reproducibility and accuracy of the grating arrays on disparate parts of the wafer.
Recently, the market for fiber optic communication has exploded and the requirements for multi channel spacing has been tightened. Because of the development of the field, more elements like filters and selective couplers have been designed that also rely on one dimensional gratings that need to be adapted to the desired wavelength. This renders the initially small market much more attractive both in number of possible applications and in volume per application. Current fine tuning requirements of DFB lasers, phaser gratings, and filters in optical communication require a tuning in steps of 100 ppm between channels or increments as small as 0.2 nm in the 1500 nm IR band. With such fine tuning the 100 nm wide IR spectral region useful for fibre optic communication could be subdivided into much more than 100 different wavelength bands or channels. Fabrication of such grating arrays directly onto the optical components will be not economic enough and requires new fabrication approaches. The next step to define each of the gratings in a master and to replicate it to the final structure using stamping or other methods is feasible but the variability of the system may require a fine-tunig during manufacturing to guarantee wavelength accuracy of the components, a process that cannot be done through fabrication of a set of masters with slightly shifted overall pitches of the arrays.
Microcontact printing (hereinafter μCP) is a technique for forming patterns of organic monolayers with micrometer and submicron lateral dimensions. It offers experimental simplicity and flexibility in forming certain types of patterns. So far, most of the prior art relies on the remarkable ability of self-assembled monolayers of long chain alkanethiolates to form on, e.g., gold or other metals. These patterns can act as nanometer resists by protecting the supporting metal from corrosion by appropriately formulated etchants, or can allow for the selective placement of fluids on hydrophilic regions of the pattern. Patterns of self-assembled monolayers having dimensions that can be less than 1 micrometer are formed by using the alkanethiol as an “ink”, and by printing them on the metal support using an elastomeric “stamp”. The stamp is fabricated by molding a silicone elastomer using a master prepared by optical lithography, by e-beam lithography, or by other techniques. Patterning of the surface of such a stamp is, e.g., disclosed in EP-B-0 784 543.
Step-and-flash-imprint lithography, also called micromolding or UV-molding, is a technique that has the potential to replace photolithography for patterning resist with sub-100 nm features. It is a low cost, high throughput alternative to conventional photolithography for high-resolution patterning. It is a molding process in which the topography of a template defines the patterns created on a substrate. This technique uses a low viscosity, photosensitive solution that is hardened by UV after having been patterned by the topographically structured quartz master (cf. M. Colburn et al., J. Vac. Sci. Technol. B, 2162 (2001)).
U.S. Pat. No. 5,817,242 discloses a hybrid stamp structure for lithographic processing of features below 1 micron. The stamp offers means for achieving a self-alignment, this means comprising key-and-lock type topographical features, e.g., cone- or pyramid-shaped protrusions and holes, which after a sufficiently accurate pre-positioning by stepping devices guide the stamp into the desired final position. Especially, the stamp comprises wedge-shaped protrusions exceeding the features of the lithographic pattern, fitting exactly into corresponding recesses of the substrate. The geometrical shape of the features thus causes a fine-adjustment of the stamp and the substrate. Furthermore, this document discloses self-alignment means based on the property or tendency of a liquid to minimize its surface. An efficient self-aligning mechanism is achieved with hydrophilic pads on the surface of both the substrate and the stamp, together with a controlled amount of moisture.
In Y. Xia et al., “Reduction in the Size of Features of Patterned SAMs Generated by Microcontact Printing with Mechanical Compression of the Stamp”, Advanced Materials, 7(1995), May, No. 5, p. 471–73, there is disclosed a method for forming patterns with submicrometer-sized features using μCP, in which the relief pattern in a polydimethylsiloxane (PDMS) stamp is first formed with relatively large features (2–10 μm) and then compressed mechanically by the application of lateral force by means of a pair (or two pairs) of small plates. However, the reproducibility of the features is limited by the system used to compress the stamp laterally using pressure applied to the plates that sandwich it. This compression is accomplished using screws.
Finally, the state of the art discloses a stamp device for printing a pattern on a surface of a substrate having a two-sided rigid carrier layer providing on its first side a patterned layer made of a first material and being combined on its second side with a soft layer made of a softer material than the first material.